Among the various video display systems available in the art, an optical projection system is known to be capable of providing high quality displays in a large scale. In such an optical projection system, light from a lamp is uniformly illuminated onto an array of, e.g., M.times.N, actuated mirrors, wherein each of the mirrors is coupled with each of the actuators. The actuators may be made of an electrodisplacive material such as a piezoelectric or an electrostrictive material which deforms in response to an electric field applied thereto.
The reflected light beam from each of the mirrors is incident upon an aperture of, e.g., an optical baffle. By applying an electric signal to each of the actuators, the relative position of each of the mirrors to the incident light beam is altered, thereby causing a deviation in the optical path of the reflected beam from each of the mirrors. As the optical path of each of the reflected beams is varied, the amount of light reflected from each of the mirrors which passes through the aperture is changed, thereby modulating the intensity of the beam. The modulated beams through the aperture are transmitted onto a projection screen via an appropriate optical device such as a projection lens, to thereby display an image thereon.
In FIGS. 1A to 1H, there are cross sectional views illustrating a method for manufacturing an array 100 of M.times.N thin film actuated mirrors 101, wherein M and N are integers, for use in an optical projection system, disclosed in a copending commonly owned application, U.S. Ser. No. 08/602,928, entitled "THIN FILM ACTUATED MIRROR ARRAY FOR USE IN AN OPTICAL PROJECTION SYSTEM".
The process for the manufacture of the array 100 begins with the preparation of an active matrix 110 including a substrate 112 and an array of M.times.N connecting terminals 114. The substrate 112 is made of an insulating material, e.g., Si-wafer, and the connecting terminal 114 is made of a conducting material, e.g., tungsten (W), as shown in FIG. 1A.
In a subsequent step, there is formed a passivation layer 120, made of, e.g., PSG or silicon nitride, and having a thickness of 0.1-2 .mu.m, on top of the active matrix 110 by using, e.g., a CVD or a spin coating method.
Thereafter, an etchant stopping layer 130, made of silicon nitride, and having a thickness of 0.1-2 .mu.m, is deposited on top of the passivation layer 120 by using, e.g., a sputtering or a CVD method, as shown in FIG. 1B.
Then, a thin film sacrificial layer 140, made of a PSG, and having a flat top surface, is formed on top of the etchant stopping layer 130 by using a CVD or spin coating method, followed by a chemical mechanical polishing (CMP) method.
Subsequently, an array of M.times.N pairs of empty cavities 145 is created in the thin film sacrificial layer 140 in such a way that one of the empty cavities 145 in each pair encompasses one of the connecting terminals 114 by using a dry or an wet etching method, as shown in FIG. 1C.
In a next step, an elastic layer 150, made of a nitride, e.g., silicon nitride, and having a thickness of 0.1-2 .mu.m, is deposited on top of the thin film sacrificial layer 140 including the empty cavities 145 by using a CVD method.
Thereafter, a second thin film layer (not shown), made of an electrically conducting material, e.g., Pt/Ta, and having a thickness of 0.1-2 .mu.m, is formed on top of the elastic layer 150 by using a sputtering or a vacuum evaporation method. The second thin film layer is then iso-cut into an array of M.times.N second thin film electrodes 165 by using a dry etching method, wherein each of the second thin film electrodes 165 is electrically disconnected from other second thin film electrodes 165, as shown in FIG. 1D.
Then, a thin film electrodisplacive layer 170, made of a piezoelectric material, e.g., PZT, or an electrostrictive material, e.g., PMN, and having a thickness of 0.1-2 .mu.m, is deposited on top of the array of M.times.N second thin film electrodes 165 by using an evaporation, a Sol-Gel, a sputtering or a CVD method.
Subsequently, a first thin film layer 180, made of an electrically conducting and light reflecting material, e.g., aluminum (Al) or silver (Ag), and having a thickness of 0.1-2 .mu.m, is formed on top of the thin film electrodisplacive layer 170 by using a sputtering or a vacuum evaporation method, thereby forming a multiple layered structure 200, as shown in FIG. 1E.
In an ensuing step, as shown in FIG. 1F, the multiple layered structure 200 is patterned by using a photolithography or a laser trimming method, until the thin film sacrificial layer 140 is exposed.
In a subsequent step, an array of M.times.N conduits 190, made of a metal, e.g., tungsten (W), is formed by using a lift-off method thereby forming an array of M.times.N actuating structures 210, wherein each of actuating structures 210 includes a first thin film electrode 185, a thin film electrodisplacive member 175, a second thin film electrode 165, an elastic member 155 and a conduit 190, the conduits 190 extending from top of the thin film electrodisplacive member 175 to top of a corresponding connecting terminal 114, as shown in FIG. 1G.
Finally, the thin film sacrificial layer 140 is removed by using an wet etching method using an etchant or a chemical, e.g., hydrogen fluoride (HF) vapor, to thereby form an array 100 of M.times.N thin film actuated mirrors 101, as shown in FIG. 1H.
There are certain deficiencies associated with the above described method for the manufacture of the array 100 of M.times.N thin film actuated mirrors 101. The connecting terminal 114, made of a conducting material, e.g., tungsten, is formed on top of the substrate 112, made of, e.g., Si-wafer. During the high temperature process in the manufacture of the thin film actuated mirrors 101, the silicon (Si) in the substrate 112 may diffuse to the connecting terminal 114 in such a way that a high resistance material, i.e., tungsten silicide, may form thereon to thereby cause the corresponding thin film actuated mirror 101 to malfunction.